Musnad Ahmad 26:668

Waki’ narrated to us, Sufyan narrated to us, from Abu Ishaq, from Khumayr ibn Malik, who said: Abdullah said: "I recited seventy surahs from the mouth of the Messenger of Allah, may Allah bless him and grant him peace. And Zayd ibn Thabit indeed has a lock of hair (i.e., he is young) regarding the Book (the Quran)."